(1) Field of the Invention
This invention relates to display apparatus. In particular the invention relates to display apparatus in which an interlaced scan video signal derived from a film source, using for example a telecine scanner, is used to address a progressively scanned spatial light modulator, for example an array of electrically deflectable mirrors. Such an array of mirrors is commonly known, when addressed with digital signals as a digital mirror device, but is also referred to as a deformable or deflectable mirror device, or DMD.
The video signals conforming to most transmission standards, including the PAL, NTSC and SECAM standards, are formatted in an interlaced scan mode intended for use with a scanned display system, for example a display system using a cathode ray tube such as in domestic television systems or in some projector systems. To produce such interlaced video signals, each image frame is divided into two fields, the first field of each image frame being designated an xe2x80x9coddxe2x80x9d field and comprising all the odd numbered horizontal lines scanned across the image frame, and the second field being designated an xe2x80x9cevenxe2x80x9d field and comprising all the even numbered lines. When the video signal is used to drive a scanned display system such as a television receiver, the two fields of each frame are displayed in the same sequence in which they were recorded, and are integrated by the eyes of the observer who will observe the reconstituted image frame.
Digital mirror devices comprise an array of deflectable mirror elements, each mirror element including a mirror surface mounted on a torsion element over a control electrode. Applying an electrical potential to each control electrode causes the associated mirror surface to pivot, thus changing the direction of light reflected from the mirror element. By application of suitable electrical address signals to the digital mirror device, selected mirror elements will reflect light in either an xe2x80x9conxe2x80x9d direction towards a projector lens for projection onto a display screen, or an xe2x80x9coffxe2x80x9d direction towards a beam dump. It is thus possible to spatially modulate a beam of light incident on the digital mirror device to produce a projected image on a display screen. The pixels of the image displayed on the display screen will be produced by light reflected from one or more of the mirror elements.
Unlike cathode ray tube based systems such as television receivers which use such interlaced address signals, display systems using digital mirror devices display a full frame of information every field interval, all the pixels representing the frame being illuminated simultaneously rather than scanned as in a cathode ray tube. In its simplest form, each pair of successive fields may be combined to produce a complete frame which may be displayed by the digital mirror device. This is commonly known as interlaced to rolling field progressive scan conversion and is illustrated in FIG. 1. As can be seen from FIG. 1, the data in each pair of successive fields is combined to produce a frame of data which is then displayed. Thus the xe2x80x9coddxe2x80x9d and xe2x80x9cevenxe2x80x9d fields of data from the first frame are combined to produce the first frame to be displayed. The second frame to be displayed comprises the xe2x80x9cevenxe2x80x9d field of the first frame, together with the xe2x80x9coddxe2x80x9d field of the second frame. The third frame to be displayed comprises the data from the xe2x80x9coddxe2x80x9d and xe2x80x9cevenxe2x80x9d fields of the second frame. The fourth frame to be displayed comprises the data from the xe2x80x9cevenxe2x80x9d field of the second frame and the xe2x80x9coddxe2x80x9d field of the third frame, and so on. By such an arrangement a xe2x80x9csmoothedxe2x80x9d image is produced.
Where such an interlaced scan video signal is used to address a digital mirror device, because the pairs of successive fields are displayed simultaneously rather than sequentially as in a cathode ray tube, in the case of images of objects which have moved between the time of recording the two fields the time lapse between, subsequent fields in each frame will create a double image.
In order to overcome this problem of double images in a camera derived signal where there has been movement of the image between successive fields, it is known to convert the interlaced scan video signal, after being digitized into pixel values for each mirror element of the digital mirror device, providing interpolated lines of pixel values by interpolating the missing lines from the most recently received field and then combining them with the most recently received field into a complete frame of pixel values. Comparison of the interpolated field with the field immediately prior to the most recent field may be employed to establish the existence of motion between successive fields and hence can be used to determine which of the preceding and interpolated fields are used to make up the complete frame. This conversion of fields to frames is known as xe2x80x9cmotion adaptive progressive scan conversionxe2x80x9d and is illustrated schematically in FIG. 2 which shows the interpolated fields used in such a progressive scan conversion where there has been movement of the image between successive fields.
(2) Description of Related Art
An example of the use of an interlaced scan video signal to address a digital mirror device after conversion to a progressive scan is shown in U.S. Pat. No. 5530482 in which a staggered mirror device pattern is used to improve the perceived horizontal resolution of the display system.
EP-A-0740468 discloses a television receiver in which an interlace to progressive scan conversion takes place in order to enable a digital mirror device to display images represented by the input interlaced signals. Interpolated fields are included to compensate for the input data which is designed for use with a scanning display, such as a cathode ray tube.
Whilst, as can be seen from the prior art, it is possible to adapt interlaced television signals for use by a progressively scanned spatial light modulator such as a digital mirror device, particular problems arise when the video signal is derived from a scanned film source. Such scanning is typically performed by a telecine scanner which is used to transfer transparencies or films onto video tape by scanning the transparency or each frame of the film with a point source of light, typically a cathode ray tube, in a raster pattern. The red, green and blue light passing through the transparency or film is detected by a respective photodetector, for example a photo multiplier for each of the three primary colours, red, green and blue in combination with appropriate filters, so as to generate an analogue signal representative of each frame of the film for each colour channel. The three colour channel signals are then converted to the appropriate luminance and colour difference signals for the relevant transmission standard. The raster scan information for each film frame is separated onto an xe2x80x9coddxe2x80x9d field containing all the odd numbered scan lines and an xe2x80x9cevenxe2x80x9d field containing all the even numbered lines. Further, depending on the setting of the telecine machine, each image frame may be transmitted xe2x80x9coddxe2x80x9d field first or xe2x80x9cevenxe2x80x9d field first.
International application WO-A-92/09172 discloses an interlaced video signal derived from a film source using a telecine apparatus for use with a display system in which the display device is a digital mirror device. The video signal is arranged to incorporate a number of flags effective to denote that the video signal has been derived from a film source, and the position of the odd fields. Using this information, the display system is able to create frames for display in which only fields of data derived from the same image frame are combined into frames and displayed. However, in practice, video signals derived from film sources using, for example, a telecine apparatus do not incorporate such flags as the video signals are designed for use with a variety of display devices in which interlaced scans do not present a problem.
It is an object of the present invention to provide a means for enabling a display apparatus using a display device such as a deformable mirror array which displays a full frame of information at every field interval using an interlaced video signal derived from a film source.
According to an aspect of the present invention there is provided a display apparatus including a spatial light modulator means for inputting an interlaced video signal produced by scanning successive image frames, the video signal comprising data signals representative of the successive image frames; means for determining from the data signals which signals have been derived from single frames of the scanned frames and producing signals representative of the frames; and a spatial light modulator drivable by said signals representative of the frames.